tdtz got it with the four compass directions plus up and down. Known to the Lakota and others as the Six Grandfathers, this was the most sacred place in the sacred Black Hills, dwelling place of the six deities that figured so prominently in the visions of Black Elk, representing the four directions, the sky and the earth, before Doane Robinson and Gutzon Borglum took over.
OK, he nailed it and I failed to notice. I do not want to use the pictures of mineral and vegetable phalli as an excuse, though. I'll just say I was making some post-Borglum contributions.